The conventional plasma processing apparatus comprises a plasma processing chamber comprising a vacuum vessel having a raw material gas inlet opening and a discharge opening and a means for supplying electromagnetic wave or the like in order to supply energy for producing plasma of a raw material gas.
The plasma processing method to be practiced using such plasma processing apparatus utilizes high energy of radicals or ions and is applicable to various desired processes including etching and film deposition by selectively deciding processing conditions such as the respective densities of the radicals and ions, the temperature of a work, etc. A method of controlling ions with electric charges is employed in order to efficiently carrying out those processes.
Specifically, there is known a method in which a medium capable of imparting energy for producing plasma in order to provide a plasma state, a high-frequency magnetic wave of 13.56 MHz is applied to the processing gas to produce plasma, by which an energy is provided for the ions to reach the work, thereby performing the control of the ions. This method can be practiced using an apparatus shown in FIG. 9. FIG. 9 is a view schematically illustrating an example of the reactive ion etching apparatus (RIE) for etching a work by producing plasma by the action of a high-frequency magnetic wave of 13.56 MHz. see, Applied Physics, vol. 51, volume 3, pp. 350 (1982))
In the reactive ion etching apparatus shown in FIG. 9, a work electrode 909 is placed in a vacuum vessel 903 through an insulator 910, and a counter electrode 920 is arranged while facing to the work electrode 909. Plasma 906 is formed in a space between the work electrode 909 and the counter electrode 920, and a work 908 on the work electrode 909 is etched by the action of the resulting plasma. Numeral reference 905 stands for a processing gas inlet opening and numeral reference 923 stands for a matching box.
Explanation is to be made about the case of etching a Si substrate using Cl.sub.2 gas in the etching process using the above apparatus shown in FIG. 9.
First, the work Si substrate 908 to be etched is positioned on the work electrode 909 and the inside of the vacuum vessel 903 is evacuated by an exhausting system to a vacuum degree of less than 10.sup.-4 Torr for example.
Then, the Cl.sub.2 gas as the etching gas is introduced into the vacuum vessel 903 through the gas inlet opening 905, and the inside of the vessel is maintained at 0.05 Torr for example. A high-frequency power from a high-frequency power source 919 is applied through the matching box 923 to the work electrode 909 to thereby produce plasma 906 between the work electrode and the counter electrode 920.
Herein, the work electrode 909 in contact with the plasma 906 is electrically isolated from earth electric potential by way of the insulator 910 and a condenser (not shown) in the matching box 923. Because of this, a negative bias voltage (this is also called a cathode drop voltage; its degree is about a maximum value Vm of the high-frequency to have been applied) is induced at the work electrode 909 due to the difference between the mobility of an electron and that of an ion. And ions are accelerated with the action of the bias voltage to collide the work 908 together with radicals, whereby the work is etched. In this case, the ion energy is controlled by adjusting the high-frequency power to be applied and varying the bias voltage.
However, in the case of using this RIE apparatus, the ion energy to reach the surface of the work has an extent of about 2 eVm (the e herein is the charge quantity of an electron) and because of this, damage caused on the work due to ions having a high energy becomes seriously problematic in the case where the surface state at the surface of the work is important. On the other hand, in the case of performing the etching while reducing the ion energy, that is, reducing the high-frequency voltage (reducing the high-frequency power) in order to prevent occurrence of such damage, there is caused a different problem that the etching rate is reduced accordingly, wherein an appropriate etching rate cannot be attained as desired. Under these circumstances, there is an increased demand for provision of such a plasma processing method that excels in the controllability, does not cause damage at the surface of a work and provides a high processing speed.
The above prior art is to perform etching by producing plasma with the application of high frequency energy to impart energy to ions. Other than this, there is known a plasma processing method in which microwave is used in order to produce plasma and a high frequency power is applied to a work electrode in order to control the energy of ion to be irradiated to a work. According to this processing method, the plasma processing can be carried out with a good controllability since the control of plasma state and the control of the energy of an ion to be impinged to the work can be independently performed on each other. An example of the apparatus for this method is described in Japanese Patent Publication Sho.56(1981)-37311. This apparatus is of the constitution which is schematically shown in FIG. 10.
In the case where the apparatus of FIG. 10 is used for etching process, microwave generated by a microwave generating device 123 (magnetron, for instance) propagates in a waveguide 101 and is absorbed by an etching gas (introduced through a leak valve 124) being controlled, for example, to less than 1 atmospheric pressure in a vessel 107 made of an insulating material which is placed in a mirror field provided by a magnetic field-causing coil 104 and a permanent magnet 127.
In a processing chamber 125, there are provided a gas inlet opening 105, a gas discharge opening 126, a work electrode 109, a work 108, said permanent magnet 127, and an insulator 130. Active ions in plasma 106 as produced are impinged into the work 108 along the mirror field to thereby etch the surface of the work 108. At this time, the work 108 and the work electrode 109 are applied with an AC voltage of a high frequency magnetic wave as shown in the figure. As its application means, there is a manner that a high frequency power source 119 is connected to an upstream coil of an air-core transformer 128 of about 1:1 in turn ratio, the work electrode 109 is connected to one end of a downstream coil, and a capacitor 129 is connected to the other end of said downstream coil. And the other end of the capacitor 129 is electrically grounded. The circumferential wall of the processing chamber 125 is provided with the insulator 130 to electrically float the work electrode 109. The work electrode 109 is made of an electrically conductive material and because of this, the high frequency voltage applied to the work electrode 109 is applied also to the work concurrently. The capacitor 129 has a capacity of about 0.1 .mu.F for example, and it functions to make the work 108 isolated from the earth in a direct current-like manner and to allow only a high frequency current to pass through the capacitor.
Therefore, the situation becomes such that a high frequency voltage negatively biased by Vf as Vs shown in FIG. 11 is applied to the work electrode 109. Ions (the ions mentioned hereinafter mean positive ions; and they are mostly monovalent) which arrive at the work (or the work electrode 109) or electrons are accelerated or decelerated due to an electric potential difference Vs--Vp with the plasma potential Vp. The mean bias value Vf is decided so that the quantity of arrival electric charges of those ions becomes equal to that of those electrons in terms of time average. The Vp herein is of a value of 10 to 20 V. As for its wave form, shown in FIG. 11 is a sine wave in terms of expediency. In practice, the wave form is somewhat varied due to nonlinear effects of the plasma.
Here, the ion energy arriving at the work 108 has an energy of e (Vf-Vp)(e herein is a charge quantity of the electron) on the average and Vp becomes greatly smaller than Vf (Vf) Vp). Because of this, the ion energy arriving at the work can be approximated to be about eVf. Ions collide the work with this average energy, whereby the work is desirably processed. The value of this energy can be properly controlled by the Vs, particularly, the power to be outputted by the high frequency power source. Such control makes it possible to conduct processing with a good controllability or to conduct high speed processing with ions having a high energy.
By the way, in general, in the case of subjecting a wafer of Si or SiO.sub.2 to plasma processing with the use of an ion energy, there is a slight occasion for the work to be damaged as long as ions having an energy of less than 100 eV are used. However, the processing speed is heightened as the energy increases, and because of this, the energy width of an ion to be used for conducting the processing at a high speed without causing damage for the work is required to be narrow. The situation for this energy width to be narrowed is schematically shown in FIG. 12. The figure illustrates the situation where the number of ions having an energy of 100 eV or nearby this is markedly large and on the other hand, the number of other ions having an energy of other value than that in the former is markedly small. And, in the case where starting at the border of certain energy for ions, deposition comes to cause in a region where the energy is low and etching comes to cause in a region where the energy is high, use of ions having an energy with a narrow energy width makes it easier to control the processing. FIG. 13 illustrates the situation where deposition or etching is caused depending upon the magnitude of the energy of an ion to be used. However, in the prior art, because the voltage to be applied to the work is varied as shown in FIG. 11, the energies of ions to be impinged into the work are distributed up to 2e at the maximum Vo--Vpo)(where Vo and Vpo are maximum values of the Vp and Vs shown in the figure). For instance, when a high frequency voltage of Vo=100 V is applied, there will be caused ions having an energy of 100 eV on the average and about 200 eV at the maximum. Therefore, the energy width for the ions is eventually spread. Because of this, there is a problem that it is impossible to constantly and perform plasma processing at a high speed and with high efficiency.